Past exam of the materials science course of the University of Cambridge / 2019 / Paper 1 / 6 / a

The nuclear fuel supplies nuclei whose nuclear fission releases energy. Most of that energy becomes heat in the fuel and neighbouring material. The reactor transfers this heat to a working fluid; the fuel's chemical form and geometry must also remain compatible with its thermal and irradiation environment.

Nuclear fuel cladding separates the fuel and most fission products from the coolant, provides mechanical containment, and transfers heat across the fuel boundary. It must retain useful mechanical integrity and corrosion resistance during irradiation while avoiding excessive neutron absorption.

Control rods absorb neutrons. Changing their insertion changes the neutron balance and hence reactor reactivity; sufficiently strong insertion makes the chain reaction subcritical. They provide control and shutdown functions. Continued decay heat means that shutdown does not remove the need for cooling.

The coolant removes heat from the fuel region and carries it to a heat exchanger or directly to a turbine system. It can also act as a neutron moderator in some reactor types, but cooling and moderation are distinct functions and need not be performed by the same material.